1. Field of Invention
This invention relates to an improved oil well pumping unit and method thereof configured for use on new or existing oil wells. This invention is for mechanical operation of the subsurface pump and replaces existing mechanical pumping units.
2. Background of the Invention
Oil wells vary in depth from a few hundred feet to over 14,000 feet. Oil is lifted from these depths by a plunger which reciprocates within a pump barrel at the bottom of the well. The plunger is driven by a sucker rod or an interconnected series of sucker rods which extend down from the surface of the oil well to the plunger.
FIG. 1 shows the prior art representing a conventional pump jack 10 for driving the sucker rod of an oil well pump. Pump jack 10 generally comprises a walking beam 12 which is connected through a polished rod 14 to an in-hole sucker rod (not shown). Walking beam 12 is pivotally supported at an intermediate position along its length by a Samson post 16, which is in turn mounted to a base frame 18. A drive crank system 20 is also mounted to base frame 18. Base frame 18 is mounted to a concrete base to rigidly locate all components relative to the oil well.
Drive crank system 20 has a rotating eccentric crank arm 24. Crank arm 24 is driven at a constant speed by an electric or gas motor in combination with a gearbox or reducer, generally designated by the reference numeral 26. Eccentric crank arm 24 rotates about a horizontal axis.
Walking beam 12 has a driven end 30 and a working end 32 on either side of its pivotal connection to Samson post 16. One or more pitman arms 34 extend from driven end 30 to a crank pin 35 positioned intermediately along outwardly extending eccentric crank arm 24. Rotation of crank arm 24 is translated by pitman arms 34 into vertical oscillation of the walking beam's driven end 30 and corresponding oscillation of working end 32.
Working end 32 of walking beam 12 has an arcuate cable track or horsehead 36. A cable 38 is connected to the top of the cable track 36. Cable 38 extends downwardly along the cable track 36 and is connected at its lower end to polished rod 14. Pivotal oscillation of walking beam 12 thus produces corresponding vertical oscillation of polished rod 14 and of the connected sucker rod. The arcuate shape of cable track 36 ensures that forces between working end 32 and polished rod 14 remain vertically aligned at all positions of walking beam 12.
The sucker rod of an oil well pump performs its work during an upward stroke, when oil is lifted from the well. No pumping is performed during the downward stroke of the sucker rod. Accordingly, a pump jack such as described above supplies force to a sucker rod primarily during its upward stroke. Relatively little force is produced on the downward stroke. To increase efficiency of a drive system counterbalance weights are utilized to store energy during the sucker rod downward stroke and to return that energy to assist in the sucker rod upward stroke.
In pump jack 10, counterbalance weights 40 are positioned at the outermost end of crank arm 24. Such weights could also be positioned on the driven end 30 of walking beam 12. However, a mechanical advantage is obtained by placing the weights outward along the crank arm from the pitman arm connection. During the downstroke of the sucker rod the driving motor must supply energy to raise weights 40 to the top of their stroke. During the sucker rod's upstroke, however, weights 40 assist the motor and gearbox since the outward end of crank arm 24 moves downward while the sucker rod moves upward. The peak energy required by the motor is therefore greatly reduced, allowing a smaller motor to be used with corresponding increases in efficiency.
Mechanical pump jacks such as described above have been used for many years and continue to be used nearly exclusively for driving oil well pumps. One reason for the popularity of such mechanical systems is their extreme simplicity. They do not involve valves, switches, or electronics and there are a minimum of moving parts. This simplicity results in reliability which is difficult to accomplish with more complex systems. Reliability is of utmost importance since oil well pumps are unattended for long periods, often being located in remote locations.
The very nature of sucker rod displacement created by a reciprocating pump jack is another apparent reason for its success. An oil well sucker rod is often over 14,000 feet long. While reciprocating, it must not only accelerate and decelerate itself, but also a 14,000 foot oil column. In addition, it must accelerate and decelerate oil within an above-surface production line, which can be as long as five miles. Forces caused by sudden acceleration of the sucker rod are therefore very significant. Any such sudden or undue acceleration can stretch and snap the sucker rod.
The pump jack described above minimizes acceleration and deceleration forces on the sucker rod by producing an approximately sinusoidal displacement at the polished rod. The sinusoidal displacement results from translation of rotary crank motion to linear motion at the polished rod. Such sinusoidal motion significantly reduces strain on the driven sucker rod.